1. Field of the Invention
The present invention relates to a diffractive optical element suitable for an optical system applied to a video camera, a digital camera, a silver-halide film camera, or the like.
2. Description of the Related Art
As a method for reducing chromatic aberration of a lens system by a combination of glass members, a method is known in which a diffractive optical element having diffraction effect is provided at a surface of a lens, or at a part of a lens system, to reduce chromatic aberration of the lens system (see SPIE Vol. 1354 International Lens Design Conference (1990), U.S. Pat. Nos. 5,044,706, 5,790,321, and 5,044,706).
The method adopting the diffractive optical element uses a physical phenomenon in which chromatic aberration caused by a light beam with a reference wavelength through a refractive surface is a reversal of chromatic aberration through a diffractive surface.
The diffractive optical element may serve as an aspherical lens by changing the period of the periodic structure. Hence, the diffractive optical element can also reduce other aberration in addition to the chromatic aberration.
In an optical system having the diffractive optical element, if major light beams with usable light wavelengths concentrate at diffraction light with a predetermined order (hereinafter, referred to as “given order” or “design order”), the intensity of diffraction light with other orders may be low. If the intensity is zero, it theoretically means that the diffraction light does not exist.
In fact, unwanted diffraction light with orders except for the design order exists. The unwanted diffraction light passes the optical system through a passage different from that of the light beam with the design order, causing flare.
To reduce the aberration using the diffractive optical element, the diffraction efficiency of the diffraction light with the design order has to be sufficiently high for the entire usable light wavelengths.
Also, it is important to carefully consider the spectral distribution of the diffraction efficiency of the diffraction light with the design order, as well as the behavior of the unwanted diffraction light with an order except for the design order.
A diffractive optical element that improves the diffraction efficiency and reduces the unwanted diffraction light is suggested (see U.S. Pat. No. 5,847,877, U.S. Patent Application Publication No. 2003-0161044, U.S. Patent Application Publication No. 2006/0171031, U.S. Pat. Nos. 6,157,488, and 6,560,019).
The diffractive optical element, disclosed in U.S. Pat. No. 5,847,877, U.S. Patent Application Publication No. 2003-0161044, or U.S. Patent Application Publication No. 2006/0171031, has two contacting diffraction gratings, and the material and height of the diffraction gratings are appropriately determined (hereinafter, such a diffractive optical element (DOE) is referred to as “contacting two-layer DOE”).
Accordingly, high diffraction efficiency of diffraction light with a desired order can be provided for wide wavelengths. It is noted that the diffraction efficiency is expressed by the ratio of the light quantity of diffraction light with an order to the light quantity of the entire transmitted light.
A diffractive optical element, disclosed in U.S. Pat. Nos. 6,157,488 or 6,560,019, has a plurality of stacked diffraction gratings, and the material and height of the diffraction gratings are appropriately determined (hereinafter, such a diffractive optical element (DOE) is referred to as “stacked DOE”).
Accordingly, high diffraction efficiency of diffraction light with any order can be provided for wide wavelengths.
U.S. Pat. No. 5,847,877 discloses a contacting two-layer DOE in which diffraction gratings made of two kinds of glass members are stacked.
U.S. Patent Application Publication No. 2003-0161044, U.S. Patent Application Publication No. 2006/0171031, or U.S. Pat. No. 6,157,488 discloses contacting two-layer DOE or a stacked DOE in which two diffraction gratings made of two kinds of UV curable materials are stacked.
U.S. Pat. No. 6,560,019 discloses a stacked DOE using a glass member and UV curable resin.
Also, a contacting two-layer DOE or a stacked DOE is known in which a material of a grating surface (diffraction grating) is the same material as a substrate on which the diffraction gratings are provided.
For a contacting two-layer DOE or a stacked DOE, it is important to appropriately determine materials of a plurality of diffraction gratings, and to appropriately determine a method of manufacturing the diffraction gratings, so as to have high diffraction efficiency and high environment resistance for wide wavelengths.
If the manufacturing method is not appropriately determined especially depending on the kind of the material; it is difficult to obtain a stacked DOE having high diffraction efficiency and high environment resistance for wide wavelengths.
U.S. Pat. No. 5,847,877 discloses an embodiment of the diffractive optical element, in which diffraction gratings made of two kinds of typical glass members are contacted, however, does not disclose a specific manufacturing method of the diffraction gratings.
To manufacture the diffraction gratings with typical glass members using a mold, since the typical glass members have a yield point temperature of 600° C. or higher, the molding temperature has to be 600° C. or higher. The durability of the mold may decrease as the molding temperature increases, and hence, the productivity may decrease.
A grating pattern may be formed by directly cutting glass, or by lithography and etching. The manufacturing method may be complicated, and hence, the productivity may decrease. In addition, U.S. Pat. No. 5,847,877 does not disclose a manufacturing method for contacting the two kinds of diffraction gratings together.
U.S. Patent Application Publication No. 2003/0161044, U.S. Patent Application Publication No. 2006/0171031, U.S. Pat. Nos. 6,157,488, or 6,560,019, discloses a manufacturing method using resin as the material of the diffraction gratings. If the resin is thermoplastic resin, a transformation point temperature thereof may be 200° C. or lower. The molding temperature may be low, and the productivity may be high. If the resin is photocurable resin, the resin may be molded with exposure of light. Hence, the productivity may be high.
As described above, the productivity can be improved by using resin for the material of the diffraction grating. However, the characteristic of resin may be changed because of heat, humidity, and ultraviolet light. The environment resistance may decrease, and the application may be restricted. Also, when a mold is used for molding, a diffraction grating made of resin may be deformed because of shrinkage when being cured.